Charles's law for a fixed amount of gas at constant pressure states that:

Charles's law: at fixed $P$ and $n$, volume is directly proportional to absolute temperature ($T$ in Kelvin): $V/T = $ constant. Double the Kelvin temperature, double the volume. The key subtlety is that $T$ must be in Kelvin, not Celsius — Charles's law fails badly if you use Celsius (a gas at $0°$C has nonzero volume). Graphically, $V$ vs $T$ (Kelvin) is a straight line passing through the origin.